When wanting to submit a flight plan, I cannot find out where to mention that I carry a PLB, which is required in some countries.

The PLB when is used on an aircraft should be classified and registered at the COSPAS as survival ELT (ELT(S) therefore is on the Flight Plan .
An ELT(S) (PLB) which is removable from an aircraft, stowed so as to facilitate its ready use in an emergency, and manually activated by survivors pilot and or passengers.
I have a PLB fix to a support in the cabin registered with hexadecimal code (same as the Transponder S) and one extra PLB in my life jacket registered at my name when I fly over the sea.